In general, a motor has a torque ripple. The torque ripple may cause velocity fluctuation or positioning error of a servomotor. Therefore, it may decrease machining accuracy of an NC machine, or vibrate a cage of an elevator to disturb the comfort of passengers, for example. The torque ripple may be caused by a motor itself (containing a transmission), or by a rotation detection sensor (i.e., rotation detection device) of the motor. The torque ripple caused by the former is attributable to insufficient machining accuracy of a stator or a rotor of the motor, eccentricity of a rotor bearing, harmonic components of a magnetic field in the motor, or insufficient assembling accuracy of the transmission. Various studies have been conducted to work out a method for reducing the torque ripple caused by the former.
JP7-129251A discloses a method that: focuses on the torque ripple generated by a speed reducer; calculates a correction signal (Tcomp=A sin(θ+α1)) where A is a torque ripple adjustment gain, θ is the rotation angle of a transmission, and α1 is an initial phase; adds the correction signal to a target torque reference in a feed-forward manner while synchronizing the correction signal with a motor rotation cycle, thereby to cancel the torque ripple.
JP11-299277A discloses a method that: focuses the fact that there is a correlation between the torque ripple and a motor rotation angle; causes a storage device to store the correlation; reads out torque ripple data corresponding to the motor rotation angle; and subtracts the ripple component from a torque command value to determine a new torque reference value.
Meanwhile, since the torque ripple caused by a rotation detection sensor appears as a motor torque ripple, the aforementioned control of a motor control device can solve the torque ripple problem in most cases. However, if an output value of the rotation detection sensor contains a ripple component associated with the rotation angle of the rotation detection target, the amplitude of the ripple increases in proportion to the angular velocity of the rotation detection target. Consequently, the angular velocity feedback gain for controlling the torque or the rotational velocity of the motor cannot be increased. This results not only in a heavy load on the control device but also in a higher device cost.
In order to solve the above problem, a method disclosed in JP 2003-83769A may be used. This method employs a ripple cancellation means that generates a canceling signal having a phase and an amplitude determined such that the canceling signal may cancel a ripple component caused by the rotation detection sensor.
As described above, with a conventional rotation detection device, various kinds of control procedures are applied to driving device or control device of a rotary machine, so that the rotary machine equipped with the rotation detection device does not exhibit a torque ripple or velocity fluctuation of a non-negligible level even if the output of the rotation detection device contains a ripple. Therefore, the drive device and the control device of the rotary machine become complicated, thereby reducing reliability and increasing the cost. The torque ripple of an electric motor may be caused not only by a ripple contained in the output of the rotation detection device, but also by various other factors such as assembling accuracy of a speed reducer, the machining accuracy of a motor itself, and harmonic components of a magnetic field. The ripple contained in the output of the rotation detection device makes it difficult to observe the torque ripple caused by the aforementioned other factors, and thus it can be said that the rotation detection device does not fully exercise its sensor function.
Further, when the ripple component of the rotation detection device is to be cancelled, it is necessary to adjust the amplitude and the phase of the output of the ripple cancellation means such that the ripple component is properly cancelled.